Recently, electronic devices that use image sensors, such as CCD arrays, etc., have also been miniaturized along with the trend to miniaturize the image sensors. For example, the miniaturization of electronic devices which use such image sensors, such as portable module cameras and WEB cameras, etc., is striking. The portable module camera is a camera used in a portable telephone, etc., and the WEB camera is a camera used for transmission of image data via the Internet. Many photographic lenses used in these small electronic devices also emphasize smallness and portability and are constructed by using only one lens element.
Recently, the performance of image sensors has improved and a large number of pixels (e.g., about 350,000 pixels) have become available for capturing even small images. Along with such a high number of image elements, higher optical performance than before is also required for photographic lenses which form the images. Thus, a problem has arisen in that a sufficient optical imaging performance cannot be obtained using a one-piece lens construction. Particularly, there is a problem in that the optical performance at the periphery of an image markedly deteriorates when using a single lens element construction. This deterioration of performance at the periphery of an image is attributed to astigmatism, i.e., a difference in imaging positions of light imaged in the sagittal image plane versus the tangential image plane.
Accordingly, it has been considered that the number of lens elements be increased to two in order to obtain a sufficient optical performance so as to match the details detectable by the large number of image elements of recently developed image sensors. The use of a negative lens element combined with a positive lens element has been tried in the past as photographic lenses of a two-piece construction. However, few of such conventional photographic lenses of two-piece construction have a sufficient compactness in overall length. Accordingly, the need for development of photographic lenses suitable for use with compact image sensors having a high number of image elements, i.e., photographic lenses which have improved optical performance without extending the overall length of the photographic lens, has increased. For example, the development of small, yet high-performance, photographic lenses which are no larger in size than about 10 cm3, including the image sensor, is desired.
The object of the invention is to provide a photographic lens which enables the overall length of the lens to be shortened and, more particularly, to provide a lens having only two lens elements and which provides an improved optical performance at the periphery of the image field as compared to a photographic lens formed of a single lens element.